Forming metals into useful products by casting is an old art, perhaps among the oldest. One type of casting technique involves the creation of a core, the size and shape of which determines the configuration of that part of the product which is devoid of metal. Stated another way, the core represents the shape of the "space" within the finished article.
One type of core is made of finely divided silica sand which is thoroughly mixed with a binder. This sand-binder mixture is formed to the proper shape and hardened. A relatively hard, robust core is a necessity since prior to actual pouring of the molten metal, the core may be subjected to a degree of physical abuse as it is handled in the foundry. The core is also subjected to thermal shock as it comes in contact with the molten metal from which the final product is made. Examples of products which are cast using cores of the type described above are cast iron automobile engine blocks, pump housings and impeller blades to name but a few.
After the sand and binder are mixed and the core formed, it must then be cured to impart those physical properties to the core which are necessary for high quality cast products. These physical qualities include core hardness, shelf life, dimensional accuracy and shakeout characteristics, among others. Such qualities can be dramatically affected by the curing process. Clearly, the process used to make cores must be closely controlled both at the core making step and in the casting step to result in cores of high quality.
Two of several curing processes are called "hot box" and "cold box" processes. In the former, an uncured core is subjected to an elevated temperature at a controlled level and for a controlled length of time. While this method of curing cores is widely and successfully used, it is extremely energy intensive. The cost of hot box curing by the use of oil or gas can be dramatically affected by the price of those fuels.
Cold box curing is also widely used to manufacturer foundry cores. With this approach, the sand and binder are combined to form a homogeneous mixture which is then shaped into a raw core. Following such shaping, the cores are confined within a closed space (called a core box) into which a gaseous curing agent is introduced. Since raw cores made of sand and binder have a degree of porosity, this curing gas penetrates through the entirety of the core to harden the binder, thereby preparing the core for the casting process. There are several binders which are suitable for gas curing processes and among them are phenolic urethane epoxy resins, furan resins, phenolic resole resins and sodium silicate. Similarly, there are a number of curing gases suitable for such processes, carbon dioxide, sulfur dioxide, methyl formate, dimethylamine (DMEA) and triethylamine (TEA) being among them. Neither the binders nor the curing gases are listed in any particular order. Further, the listed binders and curing gases are not necessarily compatible with one another. However, compatible combinations of binders and gases are well known.
For a given core, the amount of sand required and the amount of binder needed to form that sand into a usable core may be rather precisely determined. For example, in a cold box process which uses an epoxy resin cured by sulfur dioxide (known as the epoxy gas hardening or EGH process), the weight or mass of the epoxy resin binder as a percent of sand weight will be in the range of 0.6% to 1.2%, depending upon the type of sand used and the strength requirements of the core. Similarly, the weight of binder used in the TEA process is about 1.2% of the weight of the sand to be bound. Since the mass of the binder can be readily determined for a particular core, the mass of gas needed to cure the binder in that particular core can also be rather readily determined using stoichiometric data.
The stoichiometric value is the precise amount of gas needed to cure a particular quantity of binder and excludes losses. However, it is difficult if not impossible to reliably and properly cure a core using only the stoichiometric quantity of curing gas. This is so since in common core manufacturing methods, sand and binder are blown into the core box cavity using forced air and this cavity is vented to permit the air to escape. Since these vents are not closed during the gassing or curing process, some of the curing gas also escapes through them and is ineffective to cure the binder. The amount of curing gas lost is also dependent upon the pressure at which the sections of the core box are clamped together and the integrity of the edge sealing gaskets.
Notwithstanding the foregoing and even considering gas loss through the vents, across the seals and the like, foundrymen can and do very quickly learn how to achieve proper curing.
Frequently, this is done on a judgmental or experimental basis. That is, curing gas is permitted to flow to raw cores for progressively increasing times until proper curing occurs. The time required to effect such proper curing is then noted and used as the standard. For example, U.S. Pat. No. 4,540,531 (Moy) discusses a vapor generating system which effects core curing on a time basis. The pressure and temperature of the gas vapor generating vessel are controlled and a resulting density of the curing gas is assumed rather than being actually measured. If the pressure and/or temperature of the gas generating vessel stray from the desired values, undercuring or overcuring can result.
Some cold box processes use a substantially pure curing gas, the EGH process being an example. It uses sulphur dioxide in full concentration. However, it is often desirable to cure cores by using a smaller amount of curing gas blended with carrier gas to reduce the concentration of curing gas. The free radical curing (FRC) process is an example. Such processes may use a mixture of an inert carrier gas, nitrogen for example, and a curing constituent, sulphur dioxide. A concentration of sulphur dioxide of 1%-10% is normal. U.S. Pat. No. 4,112,515 (Sandow) describes how to mix the carrier gas and the curing gas or catalyst so that the ratio of the mass of the catalyst to that of the carrier gas is carefully maintained.
With many binder and curing gas combinations, "under gassing" the binder results in an undercured, unusable core which must be scrapped. On the other hand, "over-gassing" is usually not injurious to the physical properties of the core. Therefore, it is a common practice to overgas cores although the quality of cores made with some types binders can be impaired by overgassing.
Overgassing has a number of adverse effects. Perhaps the most serious of these is that even though the gas stream from the core box vent is "scrubbed" to remove possibly-harmful constituents, some of these constituents inevitably enter the environment. Yet another disadvantage of overgassing is that the cost of curing foundry cores is unnecessarily increased.
A third disadvantage of over-gassing is that when the cores are removed from the core they are often excessively permeated with the curing gas. At the least, the odor of some curing gases is unpleasant to the foundry men called upon to handle the cured cores. In an extreme case, the quantity and toxicity of the curing gas escaping from the highly permeated core may require that the foundry men wear protective clothing and breathing apparatus.
Yet another disadvantage of known systems is that the rate at which curing gas is being consumed from a storage tank may be only roughly approximated. The result is that cylinders from which curing gas is generated are either changed too frequently (thus wasting the residual liquid) or the operator is forced to replace cylinders at an undesirable point in the core manufacturing process. Control of curing gas inventory and process continuity are unnecessarily difficult to manage using conventional gas curing methods and such methods do not lend themselves well to the creation of batch processing or inventory control records.
An improved apparatus and method for gas curing foundry cores which can dramatically reduce curing gas waste, which provides precisely the proper amount of curing gas notwithstanding variations in the performance of the gas generator, which simplifies inventory and process control and which permits the creation of a written record of the way in which a particular batch of cores was cured would be an important advance in the art.